1. Field of the Invention
This invention relates to a method and apparatus for tinning and soldering metal parts of electronic components and assemblies, such as those involving integrated circuit packages, and removing excess solder therefrom, especially electronic components with frail leads and having a fine pitch thereto. In particular, the invention relates to dipping the metal parts of these electronic assemblies and components into molten solder and then rapidly removing them, whereby the metal parts are tinned and soldered without excess solder remaining on the metal parts and unwanted solder bridges therebetween.
2. Discussion of the Related Technology
Packaging techniques for electronic systems have been developed in the past in an attempt to satisfy demands for miniaturization in the semi-conductor industry. Improved methods of miniaturization in fabricating integrated circuits enables the integration of millions of transistor circuit elements onto single integrated silicone-embodied circuits, or chips, which have resulted in increased emphasis on apparatus and methods to package these circuits in space-efficient, yet reliable and mass-producible packages.
The semiconductor industry, having the ability to manufacture complex and sophisticated integrated chips, desires to take advantage of the packaging density possibilities of these integrated circuit chips. The sophisticated technology of today's integrated circuit chips has challenged the packaging abilities of conventional mechanical designs. Integrated circuit package technology has evolved in order to keep pace with complex integrated circuit chip interconnection requirements and, in addition, for efficient transfer and dissipation of heat generated by the integrated circuit chip contained therein. New methods for packaging complex integrated circuit chips have, for example, been disclosed in U.S. Pat. application Ser. No. 07/561,417, filed Aug. 1, 1990 by Burns and incorporated herein by reference for all purposes.
Fabrication of electronic systems may require connecting together hundreds of electronic components such as resistors, capacitors, transistors and integrated circuits. Presently, printed circuit boards are utilized as a structural base and interconnection system for these components. Electrical connection and partial mechanical connection may be obtained by means of soldering. Solder may be, for example, silver alloys, tin alloys and lead alloys or combinations thereof, such as tin/lead, tin/silver and tin/antimony. Solder is formulated to melt at a specific temperature. Solder may be used to attach various types of metal together and produce a low-ohmic resistance connection therebetween. Soldering of these components may be done by hand by applying a soldering iron and flux core solder to the area to be soldered. In hand soldering, each electrical connection must be soldered one solder joint at a time. Repetitively soldering each connection may unduly heat cycle the electronic part, causing failures later and, furthermore, is susceptible to inadvertent human error resulting in cold solder joints.
Wave soldering is an improvement over hand soldering when applied to a printed circuit board requiring numerous solder connections. A wave soldering station has molten solder continuously pumping through a plenum to expose un-oxidized solder. A printed circuit board and its pre-loaded components have flux supplied to the soldering area via a flux bubbler. The solder area of the printed circuit board is preheated, then the connection area of the printed circuit board is run through the molten solder and is slowly conveyed out of the solder wave and allowed to cool. A problem with wave soldering of printed circuit boards and/or complex electrical components is the possibility of excess solder buildup and resulting solder bridges between connections. This bridging problem is controllable only if there is sufficient clearance between the connections being soldered. Hot air knives have been used to remove excess solder but the high velocity air can damage the frail leads of the electronic components.
Another method is Pre-formed soldering, which uses solder and flux preformed into a shape that fits around the electronic component area to be soldered. The component to be soldered and the pre-form solder/flux are placed in an oven and heated. The solder melts and wicks around the metal parts of the electronic component, thus, making a solder connection. Soldering may also be done by electroplating a heavy coating of solder onto both parts to be connected and then applying heat to reflow the solder between these parts.
Yet another method of reflow soldering is vapor phase, which uses a special chemical that is heated to its boiling point. The boiling point of this chemical is at the correct temperature to melt the solder. The component assembly to be soldered is covered with solder paste or preformed solder and flux, then the assembly is lowered into the vapors of the boiling chemical, thus, melting the solder which reflows and makes electrical connections.
Other methods of soldering may be by infrared radiation, lasers, heat lamps, etc. All of the above methods of soldering melt and reflow solder when joining two or more metal electrical connections.
In all of the above methods of soldering, a problem exists when the component metal connections are very closely spaced together. Soldering tends to flow over metal and form a layer thereon. When the distance between metal connections is small enough the solder layer will tend to build up and bridge over between these minimally-spaced metal connections. Solder bridges cause unwanted short circuit conditions and renders the electronic component inoperative in the system. When the electronic component lead conductors are strong and large enough to withstand a blast of hot air, an air knife may be used to clean off any excess solder buildup and/or solder bridges between electrical conductors. With the newer ultra compact integrated circuit packages, however, the clearance of these metal conductors can be too small to effectively use known techniques of remedying bridging between conductors. In addition, the leads of these high density packages are extremely delicate and cannot tolerate the physical forces that result from the use of a hot air knife. Connection complexity, minimal clearances due to high packaging densities and fragile connection leads require a new way of soldering.